Research Document - 2011/110
Assessment of Scotian Shelf Snow Crab in 2010
By J.S. Choi and B.M. Zisserson
Landings in 2010 were 576 and 13,150 t for northern and southern areas of Eastern Nova Scotia (ENS), respectively and 229 t for Crab Fishing Area (CFA) 4X in 2009/2010. The associated TACs were 576, 13,200 and of 230 t, respectively. Average, non-standardized catch rates were 55.0, 102.5 and 36.0 kg trap-1, respectively. These catch rates represent a 27% decrease for N-ENS, a 14% increase in S-ENS, and a 27% increase in CFA 4X, relative to 2009.
The capture of soft-shelled crab in N-ENS declined from 6.6% in 2009 to 3.5% in 2010, This continues an improving trend in soft crab rate since 2007 in N-ENS when 111% of the landings were estimated to have been discarded as soft crab. This is largely due to a spring fishery; 91% of the total 2010 N-ENS landings were captured in the spring, when soft-shell catchability is lower. In S-ENS, soft-shell handling decreased from 16% in 2009 to 7.7% of landings in 2010. The shift towards earlier fishing seasons appears to have improved soft shell crab handling rates in S-ENS in the 2010 season. Soft-shell discard rates in CFA 4X remain very low, in large part to a fall & winter fishery. Soft-shell incidence and associated potential handling mortality will continue to be an issue that requires continued diligence and adaptability by the snow crab fleet. Bycatch of non-target species is extremely low, being estimated to be less than 0.01% and 0.93% of total snow crab landings in ENS and CFA 4X, respectively, over the past four years.
Recruitment into the fishery is expected to continue for the next 2-4 years in N-ENS as high densities of adolescents between 70 to 90 mm CW (carapace width) have been observed. The relatively stable distribution of male crab across all size classes in S-ENS may create more stable recruitment in future years, as opposed to a cyclical recruitment observed in the past in all areas. In CFA 4X, high densities of crab between 20 to 50 mm CW were observed though a distinct lack of adolescent crab between 50 and 90mm may force the reliance of the 4X fishery on movement of crab from S-ENS. The leading edge of the current recruitment pulse began entering the fishable biomass in 2007 in S-ENS; 2008 in N-ENS; and 2009 in CFA 4X. The reproductive potential of the Scotian Shelf population peaked in 2007/2008 and is now on a declining trend. Larval production should continue for another 1-2 years in N-ENS and longer in S-ENS and 4X.
The post-fishery fishable biomass of snow crab in N-ENS was estimated to be 2,810 t (95% confidence interval (CI): 2,180 to 3,780 t; relative to 2,790 t (95% CI: 2,220 to 3,840 t) in 2009. In S-ENS, the post-fishery fishable biomass of snow crab was estimated to be 48.5 × 103 t (95% CI of: 32.2 to 77.9 × 103 t) relative to 49.3 × 103 t (95% CI of: 33.2 to 79.3 × 103 t) in 2009. In CFA 4X, the pre-fishery fishable biomass was 930 t (with a 95% CI of 590 to 1,440 t), relative to 1,070 t (95% CI of 640 to 1,730 t) in 2009/2010, representing a decrease of 13%.
These positive population characteristics are tempered by a number of additional uncertainties: The influence of predation, especially upon immature and soft shelled snow crab by groundfish. Large and rapid temperature swings as they can have both direct and indirect influences upon snow crab. For example, a strong warming event can have direct deleterious effects as snow crab are cold-water stenotherms. Even the very significant cooling trend observed throughout the Scotian Shelf in 2009 with an associated habitat expansion was observed can have negative indirect consequences such as the introduction or proliferation of invasive species or disease (e.g, Hematodinium sp.) or the reduction of gross primary production. In addition to these factors, the signs of an initial return of ecological, social and economic indicators of system state in the direction of a low invertebrate dominated system adds, further uncertainty to the medium to long-term sustainability of the Scotian Shelf snow crab population.
Fishing mortality in N-ENS was estimated to be 0.19 (95% CI: 0.14, 0.24) or a harvest rate of 17.3%, unchanged relative to 2009. Good recruitment and significantly reduced soft-shell handling results in a positive outlook. Until a strong and persistent increase in fishable biomass is observed, long-term harvest rates between 10% and 20% is part of the strategy for sustainability in this fishery. A decreased or status quo harvest strategy is recommended.
Fishing mortality in S-ENS was estimated to be 0.23 (95% CI: 0.15, 0.33) or a harvest rate of 20.5%, a small increase relative to F=0.20 in 2009. Good recruitment suggests a positive outlook; however, the capture of soft shell crab remains an important issue for this fleet. Long-term harvest rates between 10% and 30% are part of the strategy for sustainability in this fishery. A decreased or status quo harvest strategy is recommended.
Fishing mortality in CFA 4X for 2009/2010 was estimated to be 0.22 (95% CI: 0.15, 0.33) or a harvest rate of 19.7%, relative to F=0.19 in 2008/2009. Long-term harvest rates between 10% and 30% are part of the strategy for sustainability in this fishery. As recruitment into the 2011/2012 season is uncertain, a decreased or status quo harvest strategy is recommended.
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